Fading in a multipath channel causes significant degradation in the quality of communications over mobile radio channels. Transmission diversity is one of the best techniques to mitigate the effects of multipath fading as well as to improve overall system performance. In particular, spatial diversity relies on the principle of transmitting signals from spatially separated transmitters via independent fading paths. In single user communications, multiple transmit and/or reception antennas may be used. However, this may be impractical or undesirable in a mobile terminal due to the small size of the terminal. To over-come this limitation, there has been proposed a user cooperation strategy that enables single-antenna mobiles to share their antennas and generate virtual multiple-antenna transmitter that allows them to achieve transmit diversity.
User cooperation is based on the fact that, when the user transmits signals to a destination receiver, neighboring users can also receive the signals because of the broadcast nature of the wireless channel. These neighboring users can then relay the signals to the receiver. A given user sees the other users as its virtual antennas and can, thus, expect the receiver to decode the information more reliably with the help of retransmissions by the other users.